1. Field of the Invention
The present invention relates to a forming belt for manufacturing a construction material such as a slate or roof tile and to a transfer belt for manufacturing a construction material.
2. Description of Related Art
A method of manufacturing a construction material such as a slate or roof tile which is a known technology is to prepare a slurry by dissolving a raw material such as cement, pearlite, gypsum, aggregate, organic fiber, inorganic fiber or asbestos in water, form it with a forming part, transfer the obtained wet material to a pressing part, press hydrate mold and remove it from a mold.
The types of the forming part are roughly divided into a cylinder type and a fordrinier type. For the formation of a construction material, in a conventional cylinder system, a woven net such as a metal net or the like is typically used as a forming belt for covering a cylinder as in the formation of a cylinder for papermaking. In a fordrinier system, a woven net is not used as a forming belt, but rather a so-called needle felt is used that is formed by needling synthetic fiber vatts on the front and rear of a base woven of monofilament or multifilament yarn so that they cross each other. This is because it is difficult to produce a construction material having a desired thickness or a desired weight, since, unlike the raw material for making paper, the raw materials for construction materials such as a slate or roof tile are mainly very fine powder materials as described above and therefore can leak through the mesh of woven net.
In the cylinder system, a woven net can be used because thin wet materials formed by scooping a raw material with a cylinder are joined together sequentially. However, in the fordrinier system, a construction material having a desired thickness or weight must be formed almost at one time and a large number of forced suction dehydraters are installed, thereby making the use of conventional woven nets impossible because the raw material leaks therethrough.
In a fordrinier type forming part of a machine for manufacturing a construction material, water is removed from a slurry through felt by a forced suction device such as a suction box as described above.
The needle felt has such an advantage that the leakage of the raw material is small and the yield is high because it is finely formed with both its front and rear sides covered with vatts. However, it has a disadvantage in that it is easily stained because the vatts are crowded in the whole direction of the z axis and the raw material can be accumulated inside the felt. Further, when a high-pressure cleaning shower is used to remove stains, the fibers of the vatt are broken and holes are easily formed. Therefore, needle felt has extremely low cleanability. The needle felt also has a serious disadvantage in that it is inferior in elongation rigidity, flexural rigidity and dimensional or attitude stability.
Since construction materials have extremely large weights, great tension must be applied to a forming belt in order to stretch the belt tightly, and the drive roll force must be transmitted so as to enable a forming belt carrying a raw material to travel smoothly.
However, since the needle felt has low elongation rigidity and experiences a large width shrinkage and a large thickness reduction which occur according to elongation, great tens ion cannot be applied and the needle felt cannot be traveled smoothly. The needle felt has another problem that slippage occurs. When slippage occurs, the abrasion of the traveling surface of the forming belt is promoted, thereby causing such problems as a reduction in service life and the stoppage of a machine due to an increase in power load, which greatly influence productivity.
Since the needle felt cannot be stretched tightly and has low flexural rigidity, it has another problem in that slack can be produced in a portion where there is nothing to support the forming belt, such as between a suction box and a carrying roll because the needle felt cannot bear the weight of the wet material and the wet material can become broken or cracked.
The needle felt has a further problem that it is compressed gradually as it is used, whereby its thickness decreases and its dehydrating power lowers accordingly.
To solve the above problems, an attempt has been made to install an inner belt having rigidity, such as a metal net, on an inner side of the needle felt. This involves an economical problem that the inner belt is additionally required and an apparatus becomes bulky and more expensive. Since the inner belt is present between the needle felt and the suction box, the leak of suction force easily occurs and it is difficult to transmit suction force to the needle felt. Therefore, to carry out a predetermined level of dehydration, the suction pressure of the suction box must be increased or the number of suction boxes must be increased with the result of a reduction in efficiency.
In a transfer belt for manufacturing a construction material which receives and joins together thin wet materials for a construction material formed by the cylinder system sequentially, only the needle felt could be used because of wet material receiving ability in the prior art. However, the transfer belt has a problem that the needle felt is stained by fine particles which get into the needle felt together with water which moves in the felt as in the fordrinier system.